1. Field of the Invention
The present invention relates to a technology for monitoring the chromatic dispersion in a high-speed optical transmission system, and in particular, to a chromatic dispersion monitoring method and a chromatic dispersion monitoring apparatus, suitable for an automatic dispersion compensation system which is required for the ensuring of transmission characteristics, the cost reduction and the realization of optical network, and an optical transmission system.
2. Description of the Related Art
In recent years, demands for the introduction of next-generation 40 Gbit/s (gigabit/second) optical transmission system have been increased, and the research and development of such an optical transmission system have been made. In a 40 Gbit/s optical transmission system, the chromatic dispersion tolerance is significantly strict, and is changed due to the temperature variation. Therefore, an automatic compensation system using a variable dispersion compensator needs to be introduced. Further, also in a 10 Gbit/s system, commercialization of which is now progressed, since the preparation of a large variety of dispersion compensated fiber (DCF) according to various transmission distances causes problems in an initial cost, a delivery schedule and the like, the introduction of the automatic dispersion compensation system is required. Further, in years to come, since the realization of an optical network for switching routes during operations of optical adding/dropping, optical cross connecting and the like shall be progressed, it is predicted that the importance of automatic dispersion compensation system which switches a chromatic dispersion compensation amount in conjunction with the route switching will become higher.
In order to realize the automatic dispersion compensation system as described above, it is necessary to establish a technology for monitoring accurately and simply the residual dispersion of an optical signal. As conventional chromatic dispersion monitoring systems, there have been proposed, for example, an error monitoring system for detecting the residual dispersion based on error information (to be specific, the number of corrected errors output from an forward error-correction circuit and the like) which is detected when a main signal light is received to be processed at a reception end of an optical transmission system, and a clock monitoring system for extracting a clock signal of a frequency corresponding to a bit rate of a main signal light, to detect the residual dispersion based on the average power of the clock signal (refer to literature 1; G. Ishikawa and H. Ooi, “Demonstration of automatic dispersion equalization in 40 Gbit/s OTDM transmission”, Proc. ECOC'98, paper WdC06, pp. 519-520, and literature 2; H. Ooi, T. Takahara, G. Ishikawa, S. Wakana, Y. Kawahata, H. Isono and N. Mitamura, “40-Gbitls WDM Automatic Dispersion Compensation with Virtually Imaged Phased Array (VIPA) Variable Dispersion Compensators”, IEICE TRANS. COMMUN., VOL. E85-B, NO. 2, pp. 463-469, 2002). Further, there has also been proposed a two-photon absorption system for receiving a main signal light utilizing a two-photon absorbing device which absorbs simultaneously two photons, to detect the residual dispersion based on an output from the two-photon absorbing device (refer to literature 3; C. Tian and S. Kinoshita, “Polarization-lndependent Waveform Monitoring with Two-Photon Absorption in Si-APD in High-Speed Transmission Systems”, ECOC 2004 We4.P.070).
However, the conventional chromatic dispersion monitoring technologies as described above cause problems as follows.
Namely, in the above error monitoring system, since it is necessary to arrange a chromatic dispersion monitor on the reception end of the main signal light, it is difficult to apply the error monitoring system to an in-line halfway along an optical fiber transmission path. Further, there is a problem in that it is hard to distinguish the chromatic dispersion from other variation factors, for example, an optical signal to noise ratio (OSNR), the polarization mode dispersion (PMD) or the like.
In the clock monitoring system, there is a problem in that since it is necessary to use a high frequency device for a light receiving element or a clock amplifier, a high cost cannot be avoided.
In the two-photon absorption system, there is a problem in that the realization of device is not easy in view of the reliability, the polarization dependence and the like, and also it is hard to ensure the sufficient monitoring sensitivity.